A control method for a variable volume pump is known in which, instead of an ordinary bleed control for controlling a hydraulic actuator speed by changing a bleed flow rate according to an operation amount of a control valve, a directional control valve of a closed center type is used, while a virtual bleed opening is set in the control valve and an area of the bleed opening (virtual bleed opening area) is changed according to the operation amount. According to the control method, a necessary pump discharge pressure is calculated using the virtual bleed opening area and a virtual bleed amount derived therefrom to perform the pump control such that the pump discharge pressure is implemented.
However, because only the virtual bleed opening is set and a negative control restriction is not assumed, a virtual negative control system is not replicated. As is generally known, the negative control system is in touch with human sensibilities, because the speed of the hydraulic actuator is low when a load is high while the speed of the hydraulic actuator is high when the load is low.